AD9048 and AD9701 redesign

[pardner]

Hi all, I need some help with a redesign. The existing design uses two AD9701 video DAC to adjust an incoming video signal's gain and offset and then it uses a AD9048 to digitize the signal and dump it on an 8-bit data bus and saved in RAM. The AD9701s are not used to generate a video signal, but a ADV7125 is used to reconstruct the digitized video for a test monitor. Both the AD9701 and the AD9048 are obsolete and I cannot find a direct replacement.

I need some help with their theory of operation.

AD9701
The AD9701 is running on -5.2V and the data sheet says that Vout (with 10%Bright) is -0.6375V. Analog suggests the AD9708 as a replacement, but how do I get -0.6375 out of a chip running on +5V and has a +1.2V internal reference? My quick fix was to put an op-amp on the output with -0.53125 gain. I just want to be sure that this is the correct behavior of the AD9701. (Also, I went with the ADV7125 because there was already one on the board, and a -0.327 gain amp stage).

AD9048
This is the one I need some help on. It is running on ±5V, Rb is connected to -2V, and the video signal is fed into Vin and RT is grounded. My problem is, I don't know what level video signal is at. Page 7 of the AD9048 datasheet shows the input ranges in the truth table: they are all negative. Analog suggests the AD9057, but it runs on a single +5V supply and an internal +2.5V reference. So I fail to see how this device would be able to sample a negative signal. Could I simple add another negative gain op-amp to switch it up to a positive signal? Here is the schematic for the AD9048:



I should also note, I do not have a functioning board available. I am working solely from schematics and a busted board. I would love to get a scope on the signals and actually see what I am getting, but I cannot.

I appreciate your help, this is the first time I have had to deal with video signals like this.

 

[keith1200rs]

It is tricky without seeing the whole circuit. What you have shown connects the NC pins of the AD9048 to VIN which is normally not a good idea.

The AD9701 is single supply ECL so runs from -5.2V and all the voltage are negative with respect to GND. I would think if you switch to the AD9708 you would run it from +5V, not try to use the -5.2V. So, your output voltage would be positive which is probably what you want in order to feed into the AD9057. You would probably need to change the opamp between the ICs to do a level shift depending on how the voltages work out but you should be able to do it all within a single 5V supply i.e. no negative voltages. You might want to use newer opamps while you are doing the redesign and get rid of what looks like an offset null adjustment (VR3).

Keith

 

[pardner]

Thank you for the reply Keith,

I've attached the whole page this time. The attachment is the original schematic that I received. I have had zero luck finding direct replaements for the AD9701 and the AD9048 and I have not had any luck finding existing stock (Rochester made an aftermarket AD9701 that is currently being used in the system, but they to are obsolete).

My fix was to replace the two AD9701 DACs with a single ADV7125 and adjust the output via negative-fraction gain opamps. Then replace the AD9048 with an AD9057 and adjust the input signal with another op-amp (see second attachment). This is my fist solo project at this company, and I get handed a system that is older than I am! I would love to get some advice.

You might want to use newer opamps while you are doing the redesign and get rid of what looks like an offset null adjustment (VR3).

The offset null is part of their set-up and tune procedure. I'm taking the "ain't broke, don't fix it approach" on this project. The expected production life of this system is 2~3 years at 50 systems a year. Since the op-amps are still available, I would expect them to be able to do a lifetime buy should they go obsolete as well.